1. Field of the Invention
The present invention relates to a method of resistance-welding workpieces of metal, especially workpieces of aluminum alloy, and a weldment which is resistance-welded by such a method.
2. Description of the Prior Art
Aluminum alloys (including aluminum) are widely used as components in aircraft, vehicles, etc. as they are highly resistant to corrosion and have good machinability. If workpieces of aluminum alloy are to be spot-welded, a large electric current needs to be supplied to the workpieces of aluminum alloy in a short period of time because the aluminum alloy has small electric resistance and large thermal conductivity. When a large electric current is supplied to the workpieces of aluminum alloy, however, electrode tips tend to be fused with the workpieces of aluminum alloy.
It has been proposed to interpose a thin film of zinc between workpieces of aluminum alloy as disclosed in Japanese patent publication No. 54-41550. According to this publication, the thin film of zinc is interposed between regions of aluminum plates which are to be resistance-welded, and then instantaneously melted in a resistance welding process to produce a high-resistance area to concentrate electric energy thereon.
Japanese patent publication No. 59-26392 discloses a thin layer of titanium, rather than a thin film of zinc, to be interposed between aluminum workpieces to be welded, and Japanese patent publication No. 59-26393 discloses a thin sheet of stainless steel, rather than a thin film of zinc, to be interposed between aluminum workpieces to be welded. Placing an aluminum alloy having a higher electric resistance than workpieces of aluminum which are to be welded between those workpieces is disclosed in Japanese laid-open patent publication No. 63-278679.
The thin film of zinc, the thin layer of titanium, the thin sheet of stainless steel, and the high-electric-resistance aluminum alloy are effective to increase the electric resistance of a local area of the aluminum workpieces, but are ineffective to lower the electric current that is necessary to be supplied to the aluminum workpieces because all the heat required to weld the aluminum workpieces is generated as Joule heat.
Among various demands in recent years is to increase a welded area with a constant electric current supplied to workpieces that are to be resistance-welded. However, such a demand cannot be met by the above conventional welding processes because they need an increased electric current to be supplied to produce an increased welded area.
Resistance-welding of workpieces made of aluminum, stainless steel, or other metals, rather than aluminum alloy, also requires the welding current to be lowered as with the resistance-welding of workpieces of aluminum alloy.